Memory device and method for writing therefor

ABSTRACT

A method for writing a memory cell in a specific write cycle is provided. The method includes the following steps: providing a first signal having a first transition edge in the specific write cycle; providing a second signal having a second transition edge in the specific write cycle, wherein the second transition edge lags behind the first transition edge; providing a first voltage level to the memory cell; and lowering the first voltage level to a second voltage level in the specific write cycle for writing the memory cell in response to the second transition edge. A memory device is also provided.

TECHNICAL FIELD

The present disclosure relates to a method for writing a memory cell,and more particularly to a method for writing a memory cell in aspecific write cycle.

BACKGROUND

The static random access memory (SRAM) cell generally includes a firstinverter, a second inverter, a first pass transistor and a second passtransistor. The first and the second inverters are cross-coupled to forma bistable latch circuit. The first pass transistor is coupled betweenthe first inverter and a first bit line. The second pass transistor iscoupled between the second inverter and a second bit line. In order toset or reset the bistable latch circuit, the first and the second passtransistors are enabled by driving a word line and accessed by drivingthe first and the second bit lines. Each of the first and the secondinverters includes a respective p-type metal oxide semiconductor (PMOS)pull-up or load transistor, a respective n-type MOS (NMOS) pull-down ordriver transistor, and a respective storage node between the respectivePMOS pull-up transistor and the respective NMOS pull-down transistor.

When the SRAM cell has a static noise margin (SNM) near zero, it mayhave a weak write property, and thus may inadvertently flip its state.The SNM is a measure of the logic circuit's tolerance to noise in eitherof the states, i.e. by how much does the input voltage change withoutdisturbing the present logic state. In other words, the SNM represents ameasure of cell robustness.

When the size of the SRAM cell is scaled down, the SRAM cell has thehuge device mismatch due to the process variation. A write operation tothe SRAM cell is enabled by asserting a desired bit value on the firstbit line and a complement of that value on the second bit line, andasserting the word line. A reduced static noise margin (SNM) can lead tocell upsets during a read operation or to unaccessed memory cells duringthe write operation. When the SRAM cell is powered by an extremely lowsupply voltage, it suffers a serious write failure due to the hugedevice mismatch.

SUMMARY

In accordance with one aspect of the present disclosure, a memory deviceis provided. The memory device includes a memory cell, a first pull-downunit and a second pull-down unit coupled to the first pull-down unit.The memory cell is to be written in a specific write cycle. The firstand the second pull-down units are sequentially switched in the specificwrite cycle.

In accordance with another aspect of the present disclosure, a methodfor writing a memory cell in a specific write cycle is provided. Themethod includes the following steps. A first signal having a firsttransition edge is provided in the specific write cycle. A second signalhaving a second transition edge is provided in the specific write cycle,wherein the second transition edge and the first transition edge are outof phase. A first voltage level is provided to the memory cell. Thefirst voltage level is lowered to a second voltage level in the specificwrite cycle for writing the memory cell in response to the first and thesecond transition edges.

In accordance with one more aspect of the present disclosure, a methodfor writing a memory cell in a specific write cycle is provided. Themethod includes the following steps. A first signal having a firsttransition edge is provided in the specific write cycle. A second signalhaving a second transition edge is provided in the specific write cycle,wherein the second transition edge lags behind the first transitionedge. A first voltage level is provided to the memory cell. The firstvoltage level is lowered to a second voltage level in the specific writecycle for writing the memory cell in response to the second transitionedge.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIG. 1A is a schematic diagram showing a memory device according to afirst embodiment of the present disclosure;

FIG. 1B is a schematic diagram showing waveforms obtained from thememory device in FIG. 1A;

FIG. 1C is a schematic diagram showing a configuration associated withthe memory cell in FIG. 1A;

FIG. 2A and FIG. 2B are schematic diagrams showing waveforms obtainedfrom the memory device in FIG. 1A;

FIG. 3 is a schematic diagram showing a memory device according to asecond embodiment of the present disclosure;

FIG. 4A and FIG. 4B are schematic diagrams showing waveforms obtainedfrom the memory device in FIG. 3;

FIG. 5 is a schematic diagram showing a second set of waveforms obtainedfrom the memory device in FIG. 3;

FIG. 6 is a schematic diagram showing a third set of waveforms obtainedfrom the memory device in FIG. 3;

FIG. 7 is a schematic diagram showing a memory device according to athird embodiment of the present disclosure; and

FIG. 8 is a flowchart showing a method for writing a memory cell of thememory device in FIG. 3 in a specific write cycle.

DETAILED DESCRIPTION

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use, and do notlimit the scope of the disclosure.

FIG. 1A is a schematic diagram showing a memory device 10 according to afirst embodiment of the present disclosure. The memory device 10includes a memory array 101 and a power supply unit 102 coupled to thememory array 101. The power supply unit 102 receives a positive supplyvoltage V1 _(DD), and converts the positive supply voltage V1 _(DD) intoa supply voltage V2 _(DD) for powering the memory array 101 in responseto an enable signal EN1.

In one embodiment, the memory array 101 includes a power line PL11, aplurality of memory cells 1011, 1012 . . . and 1016, and a plurality ofword lines WL11, WL12 . . . and WL16 coupled to the plurality of memorycells 1011, 1012 . . . and 1016, respectively. The plurality of memorycells 1011, 1012 . . . and 1016 constitute a column of the memory array101. The power line PL11 transmits the supply voltage V2 _(DD) to theplurality of memory cells 1011, 1012 . . . and 1016. For instance, thememory array 101 is an SRAM array, and the memory cell 1011 is an SRAMcell.

In one embodiment, the power supply unit 102 includes a pull-up unit1021, a pull-down unit 1031 and a node 1022 disposed between the pull-upunit 1021 and the pull-down unit 1031. The pull-up unit 1021 is activeunder the positive supply voltage V1 _(DD). For instance, the pull-upunit 1021 includes a PMOS transistor, which has a gate terminal coupledto the ground GND.

In one embodiment, the pull-down unit 1031 is switched in response tothe enable signal EN1. The enable signal EN1 has a disable level EV11,an enable level ED11 and a transition edge ET11 changed from the disablelevel EV11 to the enable level ED 11. The supply voltage V2 _(DD) may bean internal supply voltage, and be pulled up to a voltage level V21 bythe positive supply voltage V1 _(DD) when the pull-down unit 1031 isturned off in response to the disable level EV11, wherein the voltagelevel V21 is near the positive supply voltage V1 _(DD). The power supplyunit 102 lowers the supply voltage V2 _(DD) from the voltage level V21to a voltage level V22 to have a transition edge V2A when the pull-downunit 1031 is turned on in response to the transition edge ET11. Forinstance, the pull-down unit 1031 includes an NMOS transistor, which hasa gate terminal receiving the enable signal EN1.

In one embodiment, the memory device 10 is configured to make a specificwrite operation WP11 to the memory cell 1011 in a specific write cycleCW1. For the specific write operation WP11 in the specific write cycleCW1, the pull-down unit 1031 is turned on in response to the enablesignal EN1, and the memory cell 1011 is selected from the plurality ofmemory cells 1011, 1012 . . . and 1016 by driving the word line WL11with an enable signal EN2. For instance, the memory cell 1011 is aselected cell in the specific write cycle CW1, and the memory cells 1012. . . and 1016 are unselected cells therein.

In one embodiment, the memory device 10 in FIG. 1A may be in one of aplurality of conditions, wherein the plurality of conditions include afirst condition and a second condition. The memory cell 1011 has a firstweak write property when the memory device 10 is in the first condition.The memory cell 1011 has a second weak write property when the memorydevice 10 is in the second condition.

FIG. 1B is a schematic diagram showing waveforms obtained from thememory device 10 in FIG. 1A. The waveforms in FIG. 1B show the enablesignal EN1, the supply voltage V2 _(DD) (denoted to be V3 _(DD)) in thefirst condition and the supply voltage V2 _(DD) (denoted to be V4 _(DD))in the second condition, respectively.

The supply voltages V3 _(DD) and V4 _(DD) have a transition edge V3A anda transition edge V4A, respectively, each of which has a respectiveundershoot. The transition edges V3A and V4A have an undershoot US1 andan undershoot US2, respectively, and have a minimum voltage V3A1(=V22−ΔV) and a minimum voltage V4A1 (=V22−ΔV′), respectively.

FIG. 1C is a schematic diagram showing a configuration 15 associatedwith the memory cell 1011 in FIG. 1A. The configuration 15 includes thememory cell 1011, the word line WL11 and a pair PR1 of bit lines BL1 andBL1_Bar, wherein the memory array 101 includes the configuration 15. Thememory cell 1011 in FIG. 1C may have a weak write property when thememory device 10 is in a specific condition (such as the firstcondition). The memory cell 1011 includes a power input terminal 1011A,an inverter 161, an inverter 162, a pass transistor 163 and a passtransistor 164.

The inverter 161 includes a PMOS pull-up transistor 1611, an NMOSpull-down transistor 1612 and a storage node 1613 disposed between thePMOS pull-up transistor 1611 and the NMOS pull-down transistor 1612. Theinverter 162 includes a PMOS pull-up transistor 1621, an NMOS pull-downtransistor 1622 and a storage node 1623 disposed between the PMOSpull-up transistor 1621 and the NMOS pull-down transistor 1622. Each ofthe PMOS pull-up transistor 1611 and the PMOS pull-up transistor 1621receives the supply voltage V2 _(DD) from the power input terminal1011A. The storage nodes 1613 and 1623 have a node voltage VN11 and anode voltage VN12, respectively.

The memory cell 1011 has stored a data bit DB1 having a value of 0; thatis, the node voltage VN11 is in a logical low state and the node voltageVN12 is in a logical high state. The memory cell 1011 is to be written adata bit DB2 having a value of 1 in a specific write operation WP12 whenthe memory cell 1011 has stored the data bit DB1. As shown in FIG. IC,when the memory cell 1011 is written the data bit DB2, each of the PMOSpull-up transistor 1621 and the pass transistor 163 is turned on to forma current I_(A) flowing from the power input terminal 1011A through thePMOS pull-up transistor 1621 and the pass transistor 163 to the bit lineBL1_Bar. Because of the weak write property of the memory cell 1011 inthe specific condition, the supply voltage V2 _(DD) may be caused tohave an undershoot under the on-current path of the current I_(A).

The supply voltage V2 _(DD) may have a fast slew rate, and the voltagelevel V22 of the supply voltage V2 _(DD) may have an unlimited duration.In addition, the undershoot US3 of the supply voltage V2 _(DD) may becaused for the shorter bit-line length design, and may result in theretention concern for the unselected memory cells 1012 . . . and 1016.

FIG. 2A and FIG. 2B are schematic diagrams showing waveforms obtainedfrom the memory device 10 in FIG. 1A. The waveforms in FIG. 2A show theenable signal EN1 and the supply voltage V2 _(DD) for the specific writeoperation WP12 to the memory cell 1011, respectively. The waveforms inFIG. 2B show the node voltage VN11 and the node voltage VN12 (shown inFIG. 1C) for the specific write operation WP12 to the memory cell 1011,respectively. According to the waveforms in FIGS. 2A and 2B, thespecific write operation WP12 to the memory cell 1011 fails, wherein thewrite failure may result from the undershoot US3 in the supply voltageV2 _(DD).

In one embodiment, the process of the fin-type field effect transistor(FinFET) technology may make the write margin of the memory cell 1011even worse due to the Beta ratio equal to 1 and the stronger PMOSstrength of the memory cell 1011. The power-line voltage lowering of thesupply voltage V2 _(DD) may serve as a write assist since it may causethe memory device 10 to have less area consumption. However, how to keepthe voltage lowering level may be a challenge.

FIG. 3 is a schematic diagram showing a memory device 20 according to asecond embodiment of the present disclosure. In one embodiment, thememory device 20 includes a memory cell 1011, a pull-down unit 1031 anda pull-down unit 1032 coupled to the pull-down unit 1031. The memorycell 1011 is to be written in a specific write cycle CW2, and thepull-down units 1031 and 1032 are sequentially switched in the specificwrite cycle CW2.

In one embodiment, the memory device 20 includes a memory array 101 asshown in FIG. 1A and a power supply unit 202 coupled to the memory array101. In FIG. 3, the power supply unit 202 receives a positive supplyvoltage V1 _(DD), and converts the positive supply voltage V1 _(DD) intoa supply voltage V6 _(DD) for powering the memory array 101 in responseto a control signal A1, wherein the supply voltage V6 _(DD) may be aninternal supply voltage, and the control signal A1 may include aplurality of signals AN1, AN2 . . . and AN6. The power line PL11 of thememory array 101 transmits the supply voltage V6 _(DD) to the pluralityof memory cells 1011, 1012 . . . and 1016.

In one embodiment, the memory device 10 is configured to make a specificwrite operation WP21 to the memory cell 1011 in the specific write cycleCW2. For the specific write operation WP21 in the specific write cycleCW2, the supply voltage V6 _(DD) is lowered from a voltage level V61 toa voltage level V62, and the memory cell 1011 is selected from theplurality of memory cells 1011, 1012 . . . and 1016 by asserting theword line WL11 without asserting the word lines WL12 . . . and WL16. Forinstance, the memory cell 1011 is a selected cell in the specific writecycle CW2, and the memory cells 1012 . . . and 1016 are unselected cellstherein.

In one embodiment, the memory device 20 has the positive supply voltageV1 _(DD); the memory cell 1011 includes a power input terminal 1011A;and the power supply unit 202 includes a pull-up unit 1021, a node 1022and the pull-down units 1031 and 1032. The node 1022 is coupled to thepower input terminal 1011A and transmits the supply voltage V6 _(DD) tothe memory cell 1011 through the power line PL11. The pull-up unit 1021includes a terminal 1021A receiving the positive supply voltage V1 _(DD)and a terminal 1021B coupled to the node 1022, wherein the pull-downunit 1031 is connected in parallel with the pull-down unit 1032 throughthe node 1022.

In one embodiment, the pull-down units 1031 and 1032 may include twotransistors, respectively; and the two transistors may have twochannel-width-to-channel-length ratios, which may be the same ordifferent. The supply voltage V6 _(DD) may be pulled up to the voltagelevel V61 by the positive supply voltage V1 _(DD) when the pull-downunit 1031 is turned off to be in an off-state SF11 and the pull-downunit 1032 is turned off to be in an off-state SF21, wherein the voltagelevel V61 may be near the positive supply voltage V1 _(DD).

The supply voltage V6 _(DD) may be pulled down from the voltage levelV61 to the voltage level V62 for writing the memory cell 1011 when thepull-down units 1031 and 1032 are sequentially turned on in the specificwrite cycle CW2 to change the off-state SF11 into an on-state SN11 andthe off-state SF21 into an on-state SN21 sequentially. The supplyvoltage V6 _(DD) may be pulled up from the voltage level V62 to avoltage level V63 when the pull-down units 1031 and 1032 are turned onto change the on-state SN11 into an off-state SF12 and the on-state SN21into an off-state SF22 respectively.

In one embodiment, the pull-down unit 1031 receives the signal AN1having a transition edge AT11 and is turned on to be in the on-stateSN11 in response to the transition edge AT11. The pull-down unit 1032receives the signal AN2 having a transition edge AT21 lagging behind thetransition edge AT11, and is turned on to be in the on-state SN21 inresponse to the transition edge AT21. The signal AN1 may be an enablesignal, and further have a transition edge AT12 and an enable durationAS11 between the transition edges AT11 and AT12; and the signal AN2 maybe an enable signal, and further have a transition edge AT22 and anenable duration AS21 between the transition edges AT21 and AT22.

In one embodiment, the pull-down unit 1031 is turned off to be in theoff-state SF12 in response to the transition edge AT12, and thepull-down unit 1032 is turned off to be in the off-state SF22 inresponse to the transition edge AT22. The transition edge AT22 may be inone of a first state and a second state. The first state is that thetransition edge AT22 is in phase with the transition edge AT12; and thesecond state is that the transition edge AT22 leads the transition edgeAT12. For instance, the transition edges AT11 and AT21 are two risingedges, respectively, and each of the transition edges AT11 and AT21occurs in the specific write cycle CW2. For instance, the transitionedges AT12 and AT22 are two falling edges, respectively. In oneembodiment, each of the transition edges AT12 and AT22 occurs in thespecific write cycle CW2.

In one embodiment, the power supply unit 202 further includes at least apull-down unit 1036, wherein the pull-down unit 1036 is connected inparallel with the pull-down unit 1031 through the node 1022, and is tobe switched in the specific write cycle CW2 for generating the voltagelevel V62 after the pull-down units 1031 and 1032 are sequentiallyswitched. The pull-down unit 1036 receives a signal AN6 having atransition edge AT31, and is turned on in response to the transitionedge AT61 for generating the second voltage level V62. The signal AN6may be an enable signal, and further have a transition edge AT62 and anenable duration AS61 between the transition edges AT61 and AT62. Thepull-down unit 1036 is turned off in response to the transition edgeAT62 for generating the voltage level V63.

In one embodiment, in order to eliminate the undershoot, the powersupply unit 202 includes a plurality of pull-down units 1031, 1032 . . .and 1036 receiving the plurality of signals AN1, AN2 . . . and AN6,respectively, wherein the plurality of signals AN1, AN2 . . . and AN6may be a plurality of pulse signals, respectively. The plurality ofsignals AN1, AN2 . . . and AN6 respectively enable the pull-down units1031, 1032 . . . and 1036 to be turned on in the specific write cycleCW2 sequentially to lower the voltage level V61 to the voltage levelV62.

In one embodiment with reference to FIG. 1C, the memory device 20 has adata bit DB2, and writes the data bit DB2 to the memory cell 1011 in thespecific write cycle CW2 under the supply voltage V6 _(DD) by drivingthe word line WL11 and the bit lines BL1 and BL1_Bar.

FIG. 4A and FIG. 4B are schematic diagrams showing waveforms obtainedfrom the memory device 20 in FIG. 3. The waveforms in FIG. 4A show thesignal AN1, the signal AN2, the supply voltage V6 _(DD) in a firstperiod, respectively, when the memory device 20 is in a third condition.The waveforms in FIG. 4B show the supply voltage V6 _(DD) in a secondperiod, and the node voltages VN11 and VN12 (shown in FIG. 1C),respectively, when the memory device 20 is in the third condition.According to the waveforms in FIGS. 4A and 4B, a specific writeoperation to the memory cell 1011 is successful in a specific writecycle of the memory cell 1011, wherein the write success may result fromthe elimination or reduction of the undershoot in the supply voltage V6_(DD).

As shown in FIG. 4A and FIG. 4B, the power supply unit 202 lowers thesupply voltage V6 _(DD) from the voltage level V61 to the voltage levelV62 (=V61−ΔV6) in response to the transition edges AT11 and AT21.

In one embodiment, the transition edge AT21 may lag behind thetransition edge AT11 by a time interval ranging from 10 to 100picoseconds. In a specific embodiment, the time interval ranges from 20to 40 picoseconds. The signal timing control of the plurality of signalsAN1, AN2 . . . and AN6 for the memory device 20 can achieve a morereliable write assist without hurting the data retention of theunselected memory cells 1012 . . . and 1016. In one embodiment, theplurality of signals AN1, AN2 . . . and AN6 are configured to havedifferent enable-signal timing configurations to generate differentwaveforms of the supply voltage V6 _(DD) and eliminate the undershootissue.

FIG. 5 is a schematic diagram showing waveforms obtained from the memorydevice 20 in FIG. 3. The waveforms in FIG. 5 show the signal AN1, thesignal AN2, the supply voltage V6 _(DD) and the node voltages VN11 andVN12 (shown in FIG. 1C), respectively, when the memory device 20 is in afourth condition. According to the waveforms in FIG. 5, a specific writeoperation to the memory cell 1011 is successful in a specific writecycle of the memory cell 1011.

As shown in FIG. 5, the signal AN1 further has a disable level AV11, anenable level AD11 and a disable level AV12, wherein the transition edgeAT11 is changed from the disable level AV11 to the enable level AD11,and the transition edge AT12 is changed from the enable level AD11 tothe disable level AV11. The signal AN2 further has a disable level AV21,an enable level AD21 and a disable level AV22, wherein the transitionedge AT21 is changed from the disable level AV21 to the enable levelAD21, and the transition edge AT22 is changed from the enable level AD21to the disable level AV21. The transition edge AT22 may be in phase withthe transition edge AT21.

In one embodiment, the power supply unit 202 keeps the supply voltage V6_(DD) at the voltage level V61 when the pull-down unit 1031 is in theoff-state SF11 in response to the disable level AV11 and the pull-downunit 1032 is in the off-state SF21 in response to the disable levelAV21. The power supply unit 202 pulls down the supply voltage V6 _(DD)from the voltage level V61 to a voltage level V64 between the voltagelevels V61 and V62 when the pull-down unit 1031 is turned on to changethe off-state SF11 into the on-state SN11 in response to the transitionedge AT11 and the pull-down unit 1032 is in the off-state SF21. Thepower supply unit 202 keeps the supply voltage V6 _(DD) at the voltagelevel V64 for a predetermined duration DU11 when the pull-down unit 1031is in the on-state SN11 in response to the enable level AD11 and thepull-down unit 1032 is in the off-state SF21.

The power supply unit 202 pulls down the supply voltage V6 _(DD) fromthe voltage level V64 to the voltage level V62 when the pull-down unit1031 is in the on-state SN11 and the pull-down unit 1032 is turned on tochange the off-state SF21 into the on-state SN21 in response to thetransition edge AT21. The power supply unit 202 keeps the supply voltageV6 _(DD) at the voltage level V62 for a predetermined duration DU12 whenthe pull-down unit 1031 is in the on-state SN11 and the pull-down unit1032 is in the on-state SN21 in response to the enable level AD21. Thepower supply unit 202 pulls up the supply voltage V6 _(DD) from thevoltage level V62 to the voltage level V63 when the pull-down unit 1031is turned off to change the on-state SN11 into the off-state SF12 inresponse to the transition edge AT12 and the pull-down unit 1032 isturned off to change the on-state SN21 into the off-state SF22 inresponse to the transition edge AT22. For instance, the voltage levelV63 may be equal to the voltage level V61.

In one embodiment, the predetermined duration DU12 may be alower-voltage-level duration, and be limited to avoid having a dataretention failure to the memory cells 1012 . . . and 1016 in thespecific write cycle of the memory cell 1011.

FIG. 6 is a schematic diagram showing waveforms obtained from the memorydevice 20 in FIG. 3. The waveforms in FIG. 6 show the signal AN1, thesignal AN2, the supply voltage V6 _(DD) and the node voltages VN11 andVN12 (shown in FIG. 1C), respectively, when the memory device 20 is in afifth condition. According to the waveforms in FIG. 6, a specific writeoperation to the memory cell 1011 is successful in a specific writecycle of the memory cell 1011. The third set of waveforms in FIG. 6 issimilar to the second set of waveforms in FIG. 5, and thecharacteristics of the waveforms in FIG. 6 are described as follows.

The transition edge AT22 may lead the transition edge AT21. The powersupply unit 202 pulls up the supply voltage V6 _(DD) from the voltagelevel V62 to a voltage level V65 between the voltage levels V62 and V63when the pull-down unit 1032 is in the on-state SN11 in response to theenable level AD11 and the pull-down unit 1031 is turned off to changethe on-state SN21 into the off-state SF22 in response to the transitionedge AT22. The power supply unit 202 keeps the supply voltage V6 _(DD)at the voltage level V65 for a predetermined duration DU 13 when thepull-down unit 1031 is in the on-state SN1 and the pull-down unit 1032is in the off-state SF22 in response to the disable level AV22.

The power supply unit 202 pulls up the supply voltage V6 _(DD) from thevoltage level V65 to the voltage level V63 when the pull-down unit 1031is turned off to change the on-state SN11 into the off-state SF12 inresponse to the transition edge AT12 and the pull-down unit 1032 is inthe off-state SF22. For instance, the voltage level V65 may be equal tothe voltage level V64.

In one embodiment, the predetermined duration DU12 in FIG. 6 may be alower-voltage-level duration, and be limited to avoid having a dataretention failure to the memory cells 1012 . . . and 1016 in thespecific write cycle of the memory cell 1011. For instance, thepredetermined duration DU12 in FIG. 6 may be shorter than thepredetermined duration DU12 in FIG. 5. In one embodiment, differentpower-line lowering voltage-level waveforms may improve a write marginof the memory cell 1011 without hurting a data retention limitation ofthe memory cells 1012 . . . and 1016 in a specific write cycle of thememory cell 1011.

FIG. 7 is a schematic diagram showing a memory device 30 according to athird embodiment of the present disclosure. The memory device 30includes a memory array 101, a power supply unit 202 coupled to thememory array 101 and a signal generator 303 coupled to the power supplyunit 202. The signal generator 303 may be a pulse controller, andgenerate a control signal A1 in response to a clock signal CLK1, a writeenable signal WE1 and an address signal ADR1, wherein the control signalA1 may include the plurality of signals AN1, AN2 . . . and AN6. Thepower supply unit 202 provides the supply voltage V6 _(DD) to the memoryarray 101 in response to the control signal A1.

In one embodiment, the power supply unit 202 of the memory device 30 canachieve a reliable power-line lowering voltage level, which can avoid adata retention failure to the memory cells 1012 . . . and 1016 in thespecific write cycle of the memory cell 1011.

FIG. 8 is a flowchart showing a method 500 for writing a memory cell1011 of the memory device 20 in FIG. 3 in a specific write cycle CW2. Instep 502, a signal AN1 having a transition edge AT11 is provided in thespecific write cycle CW2. In step 504, a signal AN2 having a transitionedge AT21 is provided in the specific write cycle CW2, wherein thetransition edges AT21 and AT11 are out of phase. In step 506, a voltagelevel V61 is provided to the memory cell 1011. In step 508, the voltagelevel V61 is lowered to a voltage level V62 in the specific write cycleCW2 for writing the memory cell 1011 in response to the transition edgesAT11 and AT21.

In one embodiment, the voltage levels V61 and V62 occur when the memorycell 1011 is powered by a supply voltage V6 _(DD). The transition edgeAT21 lags behind the transition edge AT11. The signal AN1 further has adisable level AV11, an enable level AD11, a disable level AV12 and atransition edge AT12 changed from the enable level AD11 to the disablelevel AV12, wherein the transition edge AT11 is changed from the disablelevel AV11 to the enable level AD11. The signal AN2 further has adisable level AV21, an enable level AD21, a disable level AV22 and atransition edge AT22 changed from the enable level AD21 to the disablelevel AV22, wherein the transition edge AT21 is changed from the disablelevel AV21 to the enable level AD21. For instance, the transition edgeAT22 is in one of a first state and a second state; the first state isthat the transition edge AT22 is in phase with the transition edge AT12;and the second state is that the transition edge AT22 leads thetransition edge AT12.

In one embodiment, the method 500 further includes the following steps.A positive supply voltage V1 _(DD) is provided. The supply voltage V6_(DD) is pulled up to have the voltage level V61 by the positive supplyvoltage V1 _(DD) in response to the disable levels AV11 and AV21. Thesupply voltage V6 _(DD) is kept at a voltage level V64 between thevoltage levels V61 and V62 for a predetermined duration DU11 in responseto the enable level AD11 and the disable level AV21. The supply voltageV6 _(DD) is kept at the voltage level V62 for a predetermined durationDU12 in response to the enable levels AD11 and AD21.

In one embodiment with reference to FIG. 1C and FIG. 7, the method 500further includes the following steps. A data bit DB2, a clock signalCLK1, a write enable signal WE1 and an address signal ADR1 are provided.The signals AN1 and AN2 are generated in response to the clock signalCLK1, the write enable signal WE and the address signal ADR1. The databit DB2 is written to the memory cell 1011 in the specific write cycleCW2 under the supply voltage V6 _(DD).

In other embodiments, a method for writing a memory cell 1011 in aspecific write cycle CW2 includes the following steps. A first signalAN1 having a first transition edge AT11 is provided in the specificwrite cycle CW2. A second signal AN2 having a second transition edgeAT21 is provided in the specific write cycle CW2, wherein the secondtransition edge AT21 lags behind the first transition edge AT11. A firstvoltage level V61 is provided to the memory cell 1011. The first voltagelevel V61 is lowered to a second voltage level V62 in the specific writecycle CW2 for writing the memory cell 1011 in response to the secondtransition edge AT21. In one embodiment, the first voltage level V61 islowered to the second voltage level V62 in response to the transitionedges AT11 and AT21.

Embodiments

1. A memory device includes a memory cell, a first pull-down unit and asecond pull-down unit coupled to the first pull-down unit. The memorycell is to be written in a specific write cycle. The first and thesecond pull-down units are sequentially switched in the specific writecycle.

2. The memory device according to Embodiment 1, wherein: the memory cellincludes a power input terminal; the memory device further includes anode and a pull-up unit; the node is coupled to the power inputterminal; and the pull-up unit includes a first terminal receiving apositive supply voltage and a second terminal coupled to the node,wherein the first pull-down unit is connected in parallel with thesecond pull-down unit through the node.

3. The memory device according to Embodiments 1-2, wherein: the memorydevice has the positive supply voltage; the node transmits a firstsupply voltage to the memory cell; the first supply voltage is pulled upto a first voltage level by the positive supply voltage when the firstpull-down unit is turned off to be in a first off-state and the secondpull-down unit is turned off to be in a second off-state; and the firstsupply voltage is pulled down from the first voltage level to a secondvoltage level for writing the memory cell when the first and the secondpull-down units are sequentially turned on in the specific write cycleto change the first off-state into a first on-state and the secondoff-state into a second on-state sequentially.

4. The memory device according to Embodiments 1-3, wherein: the firstsupply voltage is kept at a third voltage level between the first andthe second voltage levels for a first predetermined duration when thefirst pull-down unit is in the first on-state and the second pull-downunit is in the first off-state; and the first supply voltage is kept atthe second voltage level for a second predetermined duration when thefirst pull-down unit is in the first on-state and the second pull-downunit is in the second on-state.

5. The memory device according to Embodiments 1-4, further comprising atleast a third pull-down unit, wherein the third pull-down unit isconnected in parallel with the first pull-down unit through the node,and is to be switched in the specific write cycle for generating thesecond voltage level after the first and the second pull-down units aresequentially switched.

6. The memory device according to Embodiments 1-5, wherein: the memorycell is a static random access memory cell; the first pull-down unitreceives a first signal having a first transition edge and is turned onin response to the first transition edge; and the second pull-down unitreceives a second signal having a second transition edge lagging behindthe first transition edge, and is turned on in response to the secondtransition edge.

7. The memory device according to Embodiments 1-6 further includes asignal generator, a word line, a first bit line and a second bit line,wherein: the signal generator generates the first and the second signalsin response to a clock signal, a write enable signal and an addresssignal; the word line is coupled to the memory cell; the first bit lineis coupled to the memory cell; and the second bit line coupled to thememory cell, wherein the memory device has a data bit, and writes thedata bit to the memory cell in the specific write cycle under the firstsupply voltage by driving the word line and the first and the second bitlines.

8. The memory device according to Embodiments 1-7, wherein: the firstsignal is a first enable signal and further has a third transition edgeand a first enable duration between the first transition edge and thethird transition edge; the second signal is a second enable signal andfurther has a fourth transition edge and a second enable durationbetween the second transition edge and the fourth transition edge; andthe fourth transition edge is in one of: a first state that the fourthtransition edge is in phase with the third transition edge; and a secondstate that the fourth transition edge leads the third transition edge.

9. A method for writing a memory cell in a specific write cycle includessteps of: providing a first signal having a first transition edge in thespecific write cycle; providing a second signal having a secondtransition edge in the specific write cycle, wherein the secondtransition edge and the first transition edge are out of phase;providing a first voltage level to the memory cell; and lowering thefirst voltage level to a second voltage level in the specific writecycle for writing the memory cell in response to the first and thesecond transition edges.

10. The method according to Embodiment 9, wherein: the first voltagelevel and the second voltage level occur when the memory cell is poweredby a first supply voltage; the second transition edge lags behind thefirst transition edge; the first signal further has a first disablelevel, a first enable level, a second disable level and a thirdtransition edge changed from the first enable level to the seconddisable level, wherein the first transition edge is changed from thefirst disable level to the first enable level; and the second signalfurther has a third disable level, a second enable level, a fourthdisable level and a fourth transition edge changed from the secondenable level to the fourth disable level, wherein the second transitionedge is changed from the third disable level to the second enable level.

11. The method according to Embodiments 9-10, further comprising stepsof: providing a positive supply voltage; and pulling up the first supplyvoltage to have the first voltage level by the positive supply voltagein response to the first and the third disable levels.

12. The method according to Embodiments 9-11, further comprising stepsof: keeping the first supply voltage at a third voltage level betweenthe first and the second voltage levels for a first predeterminedduration in response to the first enable level and the third disablelevel; and keeping the first supply voltage at the second voltage levelfor a second predetermined duration in response to the first enablelevel and the second enable level.

13. The method according to Embodiments 9-12, wherein: the fourthtransition edge is in one of: a first state that the fourth transitionedge is in phase with the third transition edge; and a second state thatthe fourth transition edge leads the third transition edge.

14. The method according to Embodiments 9-13, further comprising stepsof: providing a data bit, a clock signal, a write enable signal and anaddress signal; generating the first and the second enable signals inresponse to the clock signal, the write enable signal and the addresssignal; and writing the data bit to the memory cell in the specificwrite cycle under the first supply voltage.

15. A method for writing a memory cell in a specific write cycle,comprising steps of: providing a first signal having a first transitionedge in the specific write cycle; providing a second signal having asecond transition edge in the specific write cycle, wherein the secondtransition edge lags behind the first transition edge; providing a firstvoltage level to the memory cell; and lowering the first voltage levelto a second voltage level in the specific write cycle for writing thememory cell in response to the second transition edge.

16. The method according to Embodiment 15, wherein: the first voltagelevel and the second voltage level occur when the memory cell is poweredby a first supply voltage; the first voltage level is lowered to thesecond voltage level in response to the first and the second transitionedges; the first signal further has a first disable level, a firstenable level, a second disable level and a third transition edge changedfrom the first enable level to the second disable level, wherein thefirst transition edge is changed from the first disable level to thefirst enable level; and the second signal further has a third disablelevel, a second enable level, a fourth disable level and a fourthtransition edge changed from the second enable level to the fourthdisable level, wherein the second transition edge is changed from thethird disable level to the second enable level.

17. The method according to Embodiments 15-16, further comprising stepsof: providing a positive supply voltage; and pulling up the first supplyvoltage to have the first voltage level by the positive supply voltagein response to the first and the third disable levels.

18. The method according to Embodiments 15-17, further comprising stepsof: keeping the first supply voltage at a third voltage level betweenthe first and the second voltage levels for a first predeterminedduration in response to the first enable level and the third disablelevel; and keeping the first supply voltage at the second voltage levelfor a second predetermined duration in response to the first enablelevel and the second enable level.

19. The method according to Embodiments 15-18, wherein the fourthtransition edge is in one of: a first state that the fourth transitionedge is in phase with the third transition edge; and a second state thatthe fourth transition edge leads the third transition edge.

20. The method according to Embodiments 15-19, further comprising stepsof: providing a data bit, a clock signal, a write enable signal and anaddress signal; generating the first and the second enable signals inresponse to the clock signal, the write enable signal and the addresssignal; and writing the data bit to the memory cell in the specificwrite cycle under the first supply voltage.

A skilled person in the art will appreciate that there can be manyvariations to the embodiments of this disclosure. Although theembodiments and their features have been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade herein without departing from the spirit and scope of theembodiments. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, and composition of matter, means, methods andsteps described in the specification. As one of ordinary skill in theart will readily appreciate from the disclosed embodiments, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure.

The above method embodiment shows exemplary steps, but they are notnecessarily required to be performed in the order shown. Steps may beadded, replaced, changed order, and/or eliminated as appropriate, inaccordance with the spirit and scope of embodiment of the disclosure.Embodiments that combine different claims and/or different embodimentsare within scope of the disclosure and will be apparent to those skilledin the art after reviewing this disclosure.

What is claimed is:
 1. A memory device, comprising: a memory cell to bewritten in a specific write cycle; a first pull-down unit; and a secondpull-down unit coupled to the first pull-down unit, wherein the memorycell is coupled to the first and the second pull-down units, and thefirst and the second pull-down units are sequentially switched in thespecific write cycle.
 2. The memory device according to claim 1, whereinthe memory cell includes a power input terminal, and the memory devicefurther comprises: a node coupled to the power input terminal; and apull-up unit including a first terminal receiving a positive supplyvoltage and a second terminal coupled to the node, wherein the firstpull-down unit is connected in parallel with the second pull-down unitthrough the node.
 3. The memory device according to claim 2, wherein:the memory device has the positive supply voltage; the node transmits afirst supply voltage to the memory cell; the first supply voltage ispulled up to a first voltage level by the positive supply voltage whenthe first pull-down unit is turned off to be in a first off-state andthe second pull-down unit is turned off to be in a second off-state; andthe first supply voltage is pulled down from the first voltage level toa second voltage level for writing the memory cell when the first andthe second pull-down units are sequentially turned on in the specificwrite cycle to change the first off-state into a first on-state and thesecond off-state into a second on-state sequentially.
 4. The memorydevice according to claim 3, wherein: the first supply voltage is keptat a third voltage level between the first and the second voltage levelsfor a first predetermined duration when the first pull-down unit is inthe first on-state and the second pull-down unit is in the firstoff-state; and the first supply voltage is kept at the second voltagelevel for a second predetermined duration when the first pull-down unitis in the first on-state and the second pull-down unit is in the secondon-state.
 5. The memory device according to claim 3, further comprisingat least a third pull-down unit, wherein the third pull-down unit isconnected in parallel with the first pull-down unit through the node,and is to be switched in the specific write cycle for generating thesecond voltage level after the first and the second pull-down units aresequentially switched.
 6. The memory device according to claim 1,wherein: the memory cell is a static random access memory cell; thefirst pull-down unit receives a first signal having a first transitionedge and is turned on in response to the first transition edge; and thesecond pull-down unit receives a second signal having a secondtransition edge lagging behind the first transition edge, and is turnedon in response to the second transition edge.
 7. The memory deviceaccording to claim 6, further comprising: a signal generator generatingthe first and the second signals in response to a clock signal, a writeenable signal and an address signal; a word line coupled to the memorycell; a first bit line coupled to the memory cell; and a second bit linecoupled to the memory cell, wherein the memory device has a data bit,and writes the data bit to the memory cell in the specific write cycleunder the first supply voltage by driving the word line and the firstand the second bit lines.
 8. The memory device according to claim 6,wherein: the first signal is a first enable signal and further has athird transition edge and a first enable duration between the firsttransition edge and the third transition edge; the second signal is asecond enable signal and further has a fourth transition edge and asecond enable duration between the second transition edge and the fourthtransition edge; and the fourth transition edge is in one of: a firststate that the fourth transition edge is in phase with the thirdtransition edge; and a second state that the fourth transition edgeleads the third transition edge.
 9. A method for writing a memory cellin a specific write cycle, comprising steps of: providing a first signalhaving a first transition edge in the specific write cycle; providing asecond signal having a second transition edge in the specific writecycle, wherein the second transition edge and the first transition edgeare out of phase; providing a first voltage level to the memory cell;and lowering the first voltage level to a second voltage level in thespecific write cycle for writing the memory cell in response to thefirst and the second transition edges.
 10. The method according to claim9, wherein: the first voltage level and the second voltage level occurwhen the memory cell is powered by a first supply voltage; the secondtransition edge lags behind the first transition edge; the first signalfurther has a first disable level, a first enable level, a seconddisable level and a third transition edge changed from the first enablelevel to the second disable level, wherein the first transition edge ischanged from the first disable level to the first enable level; and thesecond signal further has a third disable level, a second enable level,a fourth disable level and a fourth transition edge changed from thesecond enable level to the fourth disable level, wherein the secondtransition edge is changed from the third disable level to the secondenable level.
 11. The method according to claim 10, further comprisingsteps of: providing a positive supply voltage; and pulling up the firstsupply voltage to have the first voltage level by the positive supplyvoltage in response to the first and the third disable levels.
 12. Themethod according to claim 10, further comprising steps of: keeping thefirst supply voltage at a third voltage level between the first and thesecond voltage levels for a first predetermined duration in response tothe first enable level and the third disable level; and keeping thefirst supply voltage at the second voltage level for a secondpredetermined duration in response to the first enable level and thesecond enable level.
 13. The method according to claim 10, wherein thefourth transition edge is in one of: a first state that the fourthtransition edge is in phase with the third transition edge; and a secondstate that the fourth transition edge leads the third transition edge.14. The method according to claim 9, further comprising steps of:providing a data bit, a clock signal, a write enable signal and anaddress signal; generating the first and the second enable signals inresponse to the clock signal, the write enable signal and the addresssignal; and writing the data bit to the memory cell in the specificwrite cycle under the first supply voltage.
 15. A method for writing amemory cell in a specific write cycle, comprising steps of: providing afirst signal having a first transition edge in the specific write cycle;providing a second signal having a second transition edge in thespecific write cycle, wherein the second transition edge lags behind thefirst transition edge; providing a first voltage level to the memorycell; and lowering the first voltage level to a second voltage level inthe specific write cycle for writing the memory cell in response to thesecond transition edge.
 16. The method according to claim 15, wherein:the first voltage level and the second voltage level occur when thememory cell is powered by a first supply voltage; the first voltagelevel is lowered to the second voltage level in response to the firstand the second transition edges; the first signal further has a firstdisable level, a first enable level, a second disable level and a thirdtransition edge changed from the first enable level to the seconddisable level, wherein the first transition edge is changed from thefirst disable level to the first enable level; and the second signalfurther has a third disable level, a second enable level, a fourthdisable level and a fourth transition edge changed from the secondenable level to the fourth disable level, wherein the second transitionedge is changed from the third disable level to the second enable level.17. The method according to claim 16, further comprising steps of:providing a positive supply voltage; and pulling up the first supplyvoltage to have the first voltage level by the positive supply voltagein response to the first and the third disable levels.
 18. The methodaccording to claim 16, further comprising steps of: keeping the firstsupply voltage at a third voltage level between the first and the secondvoltage levels for a first predetermined duration in response to thefirst enable level and the third disable level; and keeping the firstsupply voltage at the second voltage level for a second predeterminedduration in response to the first enable level and the second enablelevel.
 19. The method according to claim 16, wherein the fourthtransition edge is in one of: a first state that the fourth transitionedge is in phase with the third transition edge; and a second state thatthe fourth transition edge leads the third transition edge.
 20. Themethod according to claim 15, further comprising steps of: providing adata bit, a clock signal, a write enable signal and an address signal;generating the first and the second enable signals in response to theclock signal, the write enable signal and the address signal; andwriting the data bit to the memory cell in the specific write cycleunder the first supply voltage.